So the default value is null:
Interestingly, when I click on the field, I have noticed that little number '1' in the right hand side. If I back space, I then am not able to submit the sighting before completion.
Looks like there is an empty space.
Also my fields that have domains and are not free text behave the same. I can submit a feature without filling out the required field but if I select one of the domain values and then go back to selecting no value, then I can't submit the feature.
Looks like a bug to me?